1. Technical Field
The present invention relates generally to a digital modulating/demodulating method and apparatus for converting m-bit (e.g., 8 bits) data codes into n-bit (e.g., 15 bits) modulation codes, and vice versa. More particularly, the invention relates to an improvement on a digital modulator-demodulator system suitable for high-density data recording and transmission.
2. Background Art
Usually, when digital data is recorded in a recording medium or transmitted through a data transmission channel, a so-called channel coding is effected to match the digital data to characteristics of a data recording system or a data transmission system. Various types of modulating systems for such channel coding are known in the art. For example, Japanese Patent Second Publication Nos. 1-27510 and 5-68031, Japanese Patent First Publication Nos. 58-220213, 58-220214, 58-220215, and 61-84124 disclose conventional modulating systems. A typical one will be discussed below.
EFM (Eight to Fourteen Modulation)
This is well known as a modulating system utilized in a CD system wherein 8-bit data codes are converted into digital modulation codes each consisting of 14 channel bits. In general, these digital modulation codes are so formed as to satisfy the following requirements:
(a) Minimum length between transitions (i.e., a minimum run length) Tmin=3T (d=2) where T is a data bit interval: at least two binary digits 0s (d=2) exist between consecutive binary bits 1s.
(b) Maximum length between transitions (i.e., a maximum run length) Tmax=11T (k=10): a maximum number of binary digits 0s appearing between consecutive binary digits 1s is ten (k=10).
When two sequential blocks of digital modulation codes each containing 14 channel bits are combined to yield a digital modulation code sequence, a connection code of three bits is inserted between the two blocks, so that 8-bit data codes are practically converted into digital modulation codes each containing 17 channel bits.
Logic symbols or binary numbers of the connection code are so determined that a 1 is assigned to any one of the three bits when the condition Tmin=3T is satisfied over the two sequential blocks of the digital modulation codes to restrict the maximum length between transitions Tmax within 11T and decrease a Digital Sum Value (DSV).
FEM (Four to Eight Modulation)
The above listed publication No. 61-84124 teaches a digital modulation technique for converting 4-bit data into an 8-bit digital modulation code meeting Tmin=3T, and aims at reducing low-frequency components of modulated signals. A modulation table includes a plurality of optional bits which may be set to a logic symbol 1 or 0 to convert the digital modulation codes into time-sequential serial signals excluding a series of 1s for restricting Tmax and controlling DSV.
In the sequential conversion of the digital modulation codes into the time-sequential serial signals, if a bit at the second to the last bit position of the preceding digital modulation code represents a logic symbol 1 and a first bit of the following digital modulation code also represents a logic symbol 1, the condition Tmin=3T (d=2) is not met. Accordingly, in order to meet the condition Tmin=3T, a logic symbol 1 is assigned to the last bits of the digital modulation codes and logic symbols 0s are assigned to both the second bit to the last bit of the preceding digital modulation code and the firs bit of the following digital modulation.
This prior art technique has the advantages that data may be converted every 4 bits into digital modulation codes with a small modulation table and compact hardware. Most of conventional digital processing units process signals every byte, while the FEM system converts every 4-bit piece of one byte (=8 bits) data into 8-bit data. Therefore, the FEM system may practically be considered to be equal to a digital modulation system wherein 8-bit byte data is converted into a 16-bit digital modulation code.
In the EFM system of the above mentioned prior art techniques, the minimum length between transitions Tmin in 8-bit data will be 3.times.8/17=1.41 Tb, so that a DR (Density Ratio) of 1.41 results. In contrast, in the FEM system, the minimum length between transitions Tmin in 4-bit data will be 3.times.4/8=1.5 Tb, so that a DR of 1.5 results, which is greater than that of the EFM system and effective to produce high-density information. Note that Tb is a data bit interval prior to modulation, and has the relation of T=Tb.times.m/n where m is the number of bits of original data and n is the number of bits of a modulation code.
In recent years, however, there is an increasing need for recording and transmitting information at a further high density, and thus modulation at a great DR (i.e., a longer Tmin) is being sought.
Moreover, if there are digital modulation codes having the same DR, one having less low-frequency components in its modulation signal is useful in view of influence on a servo system and data detection.